Improve mark as read logic

[AndyScherzinger]

Resolves: #6059

According to our friend Claude:

The mark-as-read effect had two independent bugs:

• If the chat opened with the scroll position already at the bottom (e.g. only one unread message), or a new message arrived while the user was already at the bottom, the effect never re-ran since it only triggered on a change of the "at newest" state — leaving messages unmarked as read.
• If the first item in the list was a non-message entry (such as a date separator), the index-based lookup silently bailed out without advancing the read position at all.

The fix makes the effect re-evaluate whenever the message list changes, and replaces the index-based item lookup with a search that skips non-message entries and reliably resolves the newest actual message.

🚧 TODO

• test & review for a while

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔖 Capability is checked or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

[github-actions]

APK file: https://github.com/nextcloud/talk-android/actions/runs/25992339992/artifacts/7042914076
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